1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.util.Collection;
39 import java.util.Queue;
40
41 /**
42 * A {@link java.util.Queue} that additionally supports operations
43 * that wait for the queue to become non-empty when retrieving an
44 * element, and wait for space to become available in the queue when
45 * storing an element.
46 *
47 * <p>{@code BlockingQueue} methods come in four forms, with different ways
48 * of handling operations that cannot be satisfied immediately, but may be
49 * satisfied at some point in the future:
50 * one throws an exception, the second returns a special value (either
51 * {@code null} or {@code false}, depending on the operation), the third
52 * blocks the current thread indefinitely until the operation can succeed,
53 * and the fourth blocks for only a given maximum time limit before giving
54 * up. These methods are summarized in the following table:
55 *
56 * <table BORDER CELLPADDING=3 CELLSPACING=1>
57 * <caption>Summary of BlockingQueue methods</caption>
58 * <tr>
59 * <td></td>
60 * <td style="text-align:center"><em>Throws exception</em></td>
61 * <td style="text-align:center"><em>Special value</em></td>
62 * <td style="text-align:center"><em>Blocks</em></td>
63 * <td style="text-align:center"><em>Times out</em></td>
64 * </tr>
65 * <tr>
66 * <td><b>Insert</b></td>
67 * <td>{@link #add add(e)}</td>
68 * <td>{@link #offer offer(e)}</td>
69 * <td>{@link #put put(e)}</td>
70 * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
71 * </tr>
72 * <tr>
73 * <td><b>Remove</b></td>
74 * <td>{@link #remove remove()}</td>
75 * <td>{@link #poll poll()}</td>
76 * <td>{@link #take take()}</td>
77 * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
78 * </tr>
79 * <tr>
80 * <td><b>Examine</b></td>
81 * <td>{@link #element element()}</td>
82 * <td>{@link #peek peek()}</td>
83 * <td><em>not applicable</em></td>
84 * <td><em>not applicable</em></td>
85 * </tr>
86 * </table>
87 *
88 * <p>A {@code BlockingQueue} does not accept {@code null} elements.
89 * Implementations throw {@code NullPointerException} on attempts
90 * to {@code add}, {@code put} or {@code offer} a {@code null}. A
91 * {@code null} is used as a sentinel value to indicate failure of
92 * {@code poll} operations.
93 *
94 * <p>A {@code BlockingQueue} may be capacity bounded. At any given
95 * time it may have a {@code remainingCapacity} beyond which no
96 * additional elements can be {@code put} without blocking.
97 * A {@code BlockingQueue} without any intrinsic capacity constraints always
98 * reports a remaining capacity of {@code Integer.MAX_VALUE}.
99 *
100 * <p>{@code BlockingQueue} implementations are designed to be used
101 * primarily for producer-consumer queues, but additionally support
102 * the {@link java.util.Collection} interface. So, for example, it is
103 * possible to remove an arbitrary element from a queue using
104 * {@code remove(x)}. However, such operations are in general
105 * <em>not</em> performed very efficiently, and are intended for only
106 * occasional use, such as when a queued message is cancelled.
107 *
108 * <p>{@code BlockingQueue} implementations are thread-safe. All
109 * queuing methods achieve their effects atomically using internal
110 * locks or other forms of concurrency control. However, the
111 * <em>bulk</em> Collection operations {@code addAll},
112 * {@code containsAll}, {@code retainAll} and {@code removeAll} are
113 * <em>not</em> necessarily performed atomically unless specified
114 * otherwise in an implementation. So it is possible, for example, for
115 * {@code addAll(c)} to fail (throwing an exception) after adding
116 * only some of the elements in {@code c}.
117 *
118 * <p>A {@code BlockingQueue} does <em>not</em> intrinsically support
119 * any kind of "close" or "shutdown" operation to
120 * indicate that no more items will be added. The needs and usage of
121 * such features tend to be implementation-dependent. For example, a
122 * common tactic is for producers to insert special
123 * <em>end-of-stream</em> or <em>poison</em> objects, that are
124 * interpreted accordingly when taken by consumers.
125 *
126 * <p>
127 * Usage example, based on a typical producer-consumer scenario.
128 * Note that a {@code BlockingQueue} can safely be used with multiple
129 * producers and multiple consumers.
130 * <pre> {@code
131 * class Producer implements Runnable {
132 * private final BlockingQueue queue;
133 * Producer(BlockingQueue q) { queue = q; }
134 * public void run() {
135 * try {
136 * while (true) { queue.put(produce()); }
137 * } catch (InterruptedException ex) { ... handle ...}
138 * }
139 * Object produce() { ... }
140 * }
141 *
142 * class Consumer implements Runnable {
143 * private final BlockingQueue queue;
144 * Consumer(BlockingQueue q) { queue = q; }
145 * public void run() {
146 * try {
147 * while (true) { consume(queue.take()); }
148 * } catch (InterruptedException ex) { ... handle ...}
149 * }
150 * void consume(Object x) { ... }
151 * }
152 *
153 * class Setup {
154 * void main() {
155 * BlockingQueue q = new SomeQueueImplementation();
156 * Producer p = new Producer(q);
157 * Consumer c1 = new Consumer(q);
158 * Consumer c2 = new Consumer(q);
159 * new Thread(p).start();
160 * new Thread(c1).start();
161 * new Thread(c2).start();
162 * }
163 * }}</pre>
164 *
165 * <p>Memory consistency effects: As with other concurrent
166 * collections, actions in a thread prior to placing an object into a
167 * {@code BlockingQueue}
168 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
169 * actions subsequent to the access or removal of that element from
170 * the {@code BlockingQueue} in another thread.
171 *
172 * <p>This interface is a member of the
173 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
174 * Java Collections Framework</a>.
175 *
176 * @since 1.5
177 * @author Doug Lea
178 * @param <E> the type of elements held in this queue
179 */
180 public interface BlockingQueue<E> extends Queue<E> {
181 /**
182 * Inserts the specified element into this queue if it is possible to do
183 * so immediately without violating capacity restrictions, returning
184 * {@code true} upon success and throwing an
185 * {@code IllegalStateException} if no space is currently available.
186 * When using a capacity-restricted queue, it is generally preferable to
187 * use {@link #offer(Object) offer}.
188 *
189 * @param e the element to add
190 * @return {@code true} (as specified by {@link Collection#add})
191 * @throws IllegalStateException if the element cannot be added at this
192 * time due to capacity restrictions
193 * @throws ClassCastException if the class of the specified element
194 * prevents it from being added to this queue
195 * @throws NullPointerException if the specified element is null
196 * @throws IllegalArgumentException if some property of the specified
197 * element prevents it from being added to this queue
198 */
199 boolean add(E e);
200
201 /**
202 * Inserts the specified element into this queue if it is possible to do
203 * so immediately without violating capacity restrictions, returning
204 * {@code true} upon success and {@code false} if no space is currently
205 * available. When using a capacity-restricted queue, this method is
206 * generally preferable to {@link #add}, which can fail to insert an
207 * element only by throwing an exception.
208 *
209 * @param e the element to add
210 * @return {@code true} if the element was added to this queue, else
211 * {@code false}
212 * @throws ClassCastException if the class of the specified element
213 * prevents it from being added to this queue
214 * @throws NullPointerException if the specified element is null
215 * @throws IllegalArgumentException if some property of the specified
216 * element prevents it from being added to this queue
217 */
218 boolean offer(E e);
219
220 /**
221 * Inserts the specified element into this queue, waiting if necessary
222 * for space to become available.
223 *
224 * @param e the element to add
225 * @throws InterruptedException if interrupted while waiting
226 * @throws ClassCastException if the class of the specified element
227 * prevents it from being added to this queue
228 * @throws NullPointerException if the specified element is null
229 * @throws IllegalArgumentException if some property of the specified
230 * element prevents it from being added to this queue
231 */
232 void put(E e) throws InterruptedException;
233
234 /**
235 * Inserts the specified element into this queue, waiting up to the
236 * specified wait time if necessary for space to become available.
237 *
238 * @param e the element to add
239 * @param timeout how long to wait before giving up, in units of
240 * {@code unit}
241 * @param unit a {@code TimeUnit} determining how to interpret the
242 * {@code timeout} parameter
243 * @return {@code true} if successful, or {@code false} if
244 * the specified waiting time elapses before space is available
245 * @throws InterruptedException if interrupted while waiting
246 * @throws ClassCastException if the class of the specified element
247 * prevents it from being added to this queue
248 * @throws NullPointerException if the specified element is null
249 * @throws IllegalArgumentException if some property of the specified
250 * element prevents it from being added to this queue
251 */
252 boolean offer(E e, long timeout, TimeUnit unit)
253 throws InterruptedException;
254
255 /**
256 * Retrieves and removes the head of this queue, waiting if necessary
257 * until an element becomes available.
258 *
259 * @return the head of this queue
260 * @throws InterruptedException if interrupted while waiting
261 */
262 E take() throws InterruptedException;
263
264 /**
265 * Retrieves and removes the head of this queue, waiting up to the
266 * specified wait time if necessary for an element to become available.
267 *
268 * @param timeout how long to wait before giving up, in units of
269 * {@code unit}
270 * @param unit a {@code TimeUnit} determining how to interpret the
271 * {@code timeout} parameter
272 * @return the head of this queue, or {@code null} if the
273 * specified waiting time elapses before an element is available
274 * @throws InterruptedException if interrupted while waiting
275 */
276 E poll(long timeout, TimeUnit unit)
277 throws InterruptedException;
278
279 /**
280 * Returns the number of additional elements that this queue can ideally
281 * (in the absence of memory or resource constraints) accept without
282 * blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic
283 * limit.
284 *
285 * <p>Note that you <em>cannot</em> always tell if an attempt to insert
286 * an element will succeed by inspecting {@code remainingCapacity}
287 * because it may be the case that another thread is about to
288 * insert or remove an element.
289 *
290 * @return the remaining capacity
291 */
292 int remainingCapacity();
293
294 /**
295 * Removes a single instance of the specified element from this queue,
296 * if it is present. More formally, removes an element {@code e} such
297 * that {@code o.equals(e)}, if this queue contains one or more such
298 * elements.
299 * Returns {@code true} if this queue contained the specified element
300 * (or equivalently, if this queue changed as a result of the call).
301 *
302 * @param o element to be removed from this queue, if present
303 * @return {@code true} if this queue changed as a result of the call
304 * @throws ClassCastException if the class of the specified element
305 * is incompatible with this queue
306 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
307 * @throws NullPointerException if the specified element is null
308 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
309 */
310 boolean remove(Object o);
311
312 /**
313 * Returns {@code true} if this queue contains the specified element.
314 * More formally, returns {@code true} if and only if this queue contains
315 * at least one element {@code e} such that {@code o.equals(e)}.
316 *
317 * @param o object to be checked for containment in this queue
318 * @return {@code true} if this queue contains the specified element
319 * @throws ClassCastException if the class of the specified element
320 * is incompatible with this queue
321 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
322 * @throws NullPointerException if the specified element is null
323 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
324 */
325 boolean contains(Object o);
326
327 /**
328 * Removes all available elements from this queue and adds them
329 * to the given collection. This operation may be more
330 * efficient than repeatedly polling this queue. A failure
331 * encountered while attempting to add elements to
332 * collection {@code c} may result in elements being in neither,
333 * either or both collections when the associated exception is
334 * thrown. Attempts to drain a queue to itself result in
335 * {@code IllegalArgumentException}. Further, the behavior of
336 * this operation is undefined if the specified collection is
337 * modified while the operation is in progress.
338 *
339 * @param c the collection to transfer elements into
340 * @return the number of elements transferred
341 * @throws UnsupportedOperationException if addition of elements
342 * is not supported by the specified collection
343 * @throws ClassCastException if the class of an element of this queue
344 * prevents it from being added to the specified collection
345 * @throws NullPointerException if the specified collection is null
346 * @throws IllegalArgumentException if the specified collection is this
347 * queue, or some property of an element of this queue prevents
348 * it from being added to the specified collection
349 */
350 int drainTo(Collection<? super E> c);
351
352 /**
353 * Removes at most the given number of available elements from
354 * this queue and adds them to the given collection. A failure
355 * encountered while attempting to add elements to
356 * collection {@code c} may result in elements being in neither,
357 * either or both collections when the associated exception is
358 * thrown. Attempts to drain a queue to itself result in
359 * {@code IllegalArgumentException}. Further, the behavior of
360 * this operation is undefined if the specified collection is
361 * modified while the operation is in progress.
362 *
363 * @param c the collection to transfer elements into
364 * @param maxElements the maximum number of elements to transfer
365 * @return the number of elements transferred
366 * @throws UnsupportedOperationException if addition of elements
367 * is not supported by the specified collection
368 * @throws ClassCastException if the class of an element of this queue
369 * prevents it from being added to the specified collection
370 * @throws NullPointerException if the specified collection is null
371 * @throws IllegalArgumentException if the specified collection is this
372 * queue, or some property of an element of this queue prevents
373 * it from being added to the specified collection
374 */
375 int drainTo(Collection<? super E> c, int maxElements);
376 }